An ultrasound system has become an important and popular diagnostic tool since it has a wide range of applications. Specifically, due to its non-invasive and non-destructive nature, the ultrasound system has been extensively used in the medical profession. Modern high-performance ultrasound systems and techniques are commonly used to produce two or three-dimensional diagnostic images of internal features of an object (e.g., human organs).
As is known in the art, the ultrasound system may provide ultrasound images of various modes including a brightness mode (B mode) image representing reflection coefficients of the ultrasound signals reflected from the target object with a 2D (two-dimensional) image, a Doppler mode (D mode) image representing speed of a moving object with spectral Doppler by using a Doppler effect, a color Doppler mode (C mode) image representing speed of a moving object with colors by using the Doppler effect, and an elastic mode (E mode) image representing mechanical characteristics of tissues before and after applying a pressure thereto. In particular, the ultrasound system may transmit and receive ultrasound signals to and from the target object to thereby form Doppler signals. The ultrasound system may further form the C mode image representing the speed of the moving object with the colors based on the Doppler signals.
The Doppler signal may include a low frequency signal (so-called clutter signal) due to the motion of a tissue such as a blood vessel wall, a cardiac wall, a heart valve and the like. The clutter signal may have amplitude, which is over 100 times than that of a pure Doppler signal indicative of velocities of the blood flow. The clutter signal may be an obstacle to accurately detect speed of the blood flow. Thus, it is required to remove the clutter signal from the Doppler signal in order to accurately detect the speed of the blood flow.
A clutter downmixing has been used to remove the clutter signals. The downmixing may be carried out by estimating a center frequency of the clutter signals and by performing the downmixing (frequency shift), which shifts the center frequency of the clutter signals to 0, upon the Doppler signals based on the center frequency. Thereafter, the downmixed clutter signals, which are shifted to a DC component, may be removed by using the conventional clutter filtering methods.
The clutter signals may have a speed component when a tissue exists within the target object, wherein the tissue moves at a constant speed. However, the clutter signals may have two speed components when tissues exist within the target object, wherein the tissues comprise a tissue that does not move within the target object and a tissue that moves at the constant speed. Thus, it may be difficult to accurately perform the downmixing, which shifts the center of the clutter signals to 0, by using the conventional clutter downmixing methods. Further, the clutter signals may have a plurality of speed components when a tissue exists within the target object, wherein the tissue moves at a lower speed than a predetermined speed (e.g., minimum speed of blood flow). As such, it may be difficult to accurately perform the downmixing, which shifts the center of the clutter signals to 0, by using the conventional clutter downmixing methods.